On the strength of recent rapid development of electronics, communications, computer industry, etc., the use of portable electronic devices such as camcorders, mobile phones, notebook PCs and the like becomes generalized. Accordingly, there is increasing demand for batteries which are lightweight and highly reliable, and can be used longer.
In particular, lithium secondary batteries, whose operating voltage is 3.7 V or more, have higher energy density per unit weight than nickel-cadmium batteries and nickel-hydrogen batteries. Accordingly, the demand for the lithium secondary batteries as a power source to drive the portable electronic communication devices is increasing day by day.
Recently, studies on power sources for electric vehicles by hybridizing an internal combustion engine and a lithium secondary battery are actively conducted in the United States, Japan, Europe and the like. The development of a plug-in hybrid (P-HEV) battery used in the car with a mileage of less than 60 miles is actively proceeding around United States. The P-HEV battery is a battery having characteristics, which are nearly the characteristics of an electric vehicle, and the biggest challenge is to develop high-capacity batteries. In particular, the biggest challenge is to develop cathode materials having higher tap density of 2.0 g/cc or more and high capacity characteristics of 230 mAh/g or more.
The materials, which are currently available or under development, are LiCoO2, LiNiO2, LiMnO2, LiMn2O4, Li1+X[Mn2-xMx]O4, LiFePO4 and the like. Of them, the LiCoO2 is an excellent material having stable charge/discharge characteristics, excellent electronic conductivity, high cell voltage, high stability and even discharge voltage characteristics. However, because Co has low reserves and is expensive and toxic to the human body, it is needed to develop other cathode materials. Further, it has a defect of very poor thermal properties by unstable crystal structure by delithiation during discharging.
In order to improve it, there may be many attempts to shift the exothermic onset temperature to the side of the higher temperature and to make an exothermic peak broad in order to prevent rapid heat-emitting, by substitute a part of the nickel with transition metals. However, there is no satisfactory result yet.
Namely, LiNi1-xCoxO2 (x=0.1-0.3) material, wherein a part of the nickel is substituted with cobalt, shows excellent charge/discharge characteristics and lifetime characteristics, but the thermostability problem is not solved yet. Furthermore, European Patent No. 0872450 discloses LiaCobMncMdNi1-(b+c+d)O2 (M=B, Al, Si. Fe, Cr, Cu, Zn, W, Ti, Ga)-type, where the Ni is substituted with other metals as well as Co and Mn, but the thermostability of the Ni-based material is not solved yet.
In order to eliminate these shortcomings, Korean Patent Publication No. 2005-0083869 suggests lithium-transition metal oxides having metal composition representing concentration gradient. This method is a method that an internal materials with a certain composition is synthesized and materials with other composition is coated on the exterior thereof to obtain a bi-layer, and is mixed with a lithium salt followed by heat-treatment. The internal material may be commercially available lithium transition metal oxides. However, in this method, the metal composition of the cathode active material between the produced internal material and the external material is discontinuously changed, and is not continuously and gradually changed. Further, the powder synthesized by the invention, which does not use ammonia as a chelating agent, was not suitable for a cathode active material for a lithium secondary battery due to its lower tap density.
In order to improve this problem, Korean Patent Publication No. 2007-0097923 suggests a cathode active material, which has an internal bulk part and an external bulk part, and the metal ingredients have continuous concentration distribution depending on their position at the external bulk part. However, in this method, there was a need to develop a cathode active material of a new structure having better stability and capacity because the concentration is constant at the internal bulk part and the metal composition is changed at the external bulk part.